[Robert A Cook PE]

I have long been suspicious of the curiously-circular edges of Lakes Michigan-Huron, with the Michigan peninsula showing up as the rebounding central mountain range typical of craters.

[frithguild]

Saginaw Impact hypothesis http://cintos.org/SaginawManifold/Saginaw_Bay/index.html

[Cold Heart]

“I have long been suspicious of the curiously-circular edges of Lakes Michigan-Huron,..”

Educational program on TV about salt mining under the great lakes. The salt mine deposit is circular under the area you describe. It is immense. No mention of the cause for that in the program.